The OSI (Open System Interconnection) model is an abstract description of a layered communication network. It divides networks into seven layers which are listed below from top to down: Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data-link Layer and Physical Layer.
One of the most important functions of the Physical Layer of the OSI model is Modulation. Here, modulation refers to a technique in which a periodic high-frequency sinusoid waveform is generated as a carrier signal which is used to convey a message. Based on a difference of carriers, modulation technology can be divided into Mono-Carrier modulation and Multi-Carrier (MC) modulation. The MC modulation is to split the transmitted data into several components and to send each of these components over separate carrier signals.
The Data-link Layer can be divided into two sub-layers: MAC (Medium Access Control) Layer and LLC (Logic Link Control) Layer. The MAC layer provides an addressing mechanism and a channel access control mechanism that can make several users connect to a multipoint network and share the capacity of a physical medium. There are three different kinds of channel access control methods: circuit mode method, packet mode method and duplex method.
One of the techniques using the circuit mode method is Time Division Multiple Access (TDMA). The aim of TDMA techniques is to maximize the number of transponders able to operate simultaneously within a certain bandwidth. TDMA allows several users to send their signals in the same frequency channel by dividing the channel into different time slots. Each user uses his own series of time slots one after another. The user only needs to listen to and broadcast in its own time slots. For the rest of the time, the user can monitor the network and detect surrounding transmitters on different frequencies.
Another technique using the circuit mode method is Frequency Division Multiple Access (FDMA). The aim of FDMA techniques is to give each user an individual allocation of one or more frequency bands which are available for the entire period of communication. Therefore, a continuous flow of data without packetizing can easily be used with FDMA.
One of the techniques using the duplex method is Time Division Duplex (TDD). The terminology “duplex” is used to describe a system composed of two parties or devices which can communicate with one another in both the uplink (UL) direction and downlink (DL) direction. Full-duplex allows communication in both directions to happen simultaneously. TDD is a kind of full-duplex method. When using the TDD method, a single frequency channel is allocated to both the transmitter and the receiver. Both the uplink (UL) and downlink (DL) traffic use the same radio frequency but at different time slots.
The DECT (Digital Enhanced Cordless Telecommunication) standard has been developed by members of European Telecommunication Standards Institute (ETSI) and aims to offer services such as cordless voice, fax, data and multimedia communications, via wireless local area networks, etc. Now, DECT has become the major standard of cordless telephone communication and is used worldwide.
DECT provides various radio access methods: Frequency Division Multiple Access (FDMA), Time Division Multiple Access, and Time Division Duplex (FDMA/TDMA/TDD). The DECT system has a total of ten possible carrier frequencies (MC) with various spectral bands depending on location. European DECT frequencies range from 1880 to 1900 MHz, Chinese DECT from 1910 to 1930 MHz, Latin America from 1920 to 1930 MHz and US DECT from 1920 to 1930 MHz. The size of a timeframe in each carrier frequency is 10 ms and each timeframe comprises 24 timeslots (TDMA). The first 12 timeslots are available for downlink transmission and the other 12 timeslots are available for uplink transmission (TDD).
FIG. 1 shows a simplified, schematic view of a DECT system. The DECT system comprises one or more base stations, here called “fixed part” FP, a data monitor PP1 (or sensor), a cordless mobile phone PP2 and a remote controller PP3. Other types of nodes may be provided as well.
The DECT system is a micro-cell system comprising one or more base stations or Fixed Parts (FP) and one or more Portable terminals (or Portable Part (PP)). In each cell system there is a base station FP which may serve several portable terminals PP. In a TDMA system the Fixed Part FP and the Portable Part PP maintain a common time base to synchronize communication. A designated node in a TDMA system maintains a central time base to which the other nodes in the network synchronize. Each node can then be allotted a time slot to transmit as indicated in the lower part of FIG. 1. One way to do this is to let the Fixed Part FP broadcast a network message at predefined intervals and to let the other nodes listen to this message. The broadcast message is then used by various nodes or Portable Parts PP in the network to deduce timing information and also for instance which slots are available for transmission. Once the Portable Part PP successfully receives a number of broadcast messages from the Fixed Part FP the Portable Part PP can deduce the correct time base. Once the Portable Part PP time base is aligned to the time base of the Fixed Part FP the Portable Part PP is said to be synchronized to the Fixed Part FP. Once a link is successfully negotiated the Portable Part PP is assigned a downlink and uplink slot combination to communicate with the Fixed Part FP.
The lower part of FIG. 1 shows an example of how communication can take place in a DECT system. The fixed part FP broadcasts the network message, also called a bearer in a DECT system, in slot S0. In the example of FIG. 1 the Portable Parts PP1, PP2 and PP3 have negotiated a link and have been allotted different time slots to communicate with the Fixed Part FP. The Fixed Part FP first transmits data in slot S2 to the portable part PP1, then listens to a return message twelve slots later in slot S14. The Fixed Part FP then transmits data to portable part PP2 in slot S4 and listens to a return message from portable part PP2 in slot S16, etc. The bearer and negotiated links may be allotted to each slot, in a negotiated link the downlink slot number “x” is always combined with an uplink slot number “x+12” where “x” can be any number from 0 to 11. After a certain number of slots, i.e., equal to the length of a frame, each portable part has a new time slot to communicate with the fixed part FP. If all available slots are used, no new communication session with another portable part PP can be setup.
The architecture of a typical DECT Protocol is closely related to the OSI layer Structure, the table below shows equivalent functions of the OSI and DECT protocol:
OSI modelDECT protocolOSI Layer 3Network Layer (NWL)OSI Layer 2Data Link Control Layer (DLC)OSI Layer 2Media Access Control Layer (MAC)OSI Layer 1Physical Layer (PHY)
As is shown in FIG. 2 of the drawing, a typical, prior art one-chip RF TDMA node contains a radio frontend 1 for RF (Radio Frequency) communication, a TDMA processor 2 for controlling the Media Access, a microprocessor (uc) 5 for running a protocol stack and controlling the system and peripherals 7, a main system supply 6 which supplies power to the whole system, a memory 4 and a clock 3. For the sake of starting data transmission on the correct moment, the time base is maintained by various timers included in said TDMA processor 2. The DECT Portable Part hardware may be implemented as follows:                A. The Radio Frontend 1 which comprises a small antenna may be operated on the PHY layer for the RF communication;        B. The TDMA processor 2 may be operated on the MAC layer, it may comprise several components like (not shown):                    A Programming Component for preparing signals to get programmed on a specific carrier and slot.            A CRC coding component for generating CRC (cyclic redundancy checking) codes for error correction.            A Scrambling Component for multiplying data by a predefined stream for data whitening.            An Encryption Component for data encryption, for instance by multiplying data by a secret stream for security.                        C. The microprocessor (uC) 5 runs the protocol stack, i.e. the Network Layer and higher layers, and controls the system and peripherals 7 for interfacing. It is operated on the Network Layer and higher layers, and may comprise several components (not shown):                    Voice processing component: ADPCM (ADaptive Pulse Code Modulation). It is observed that the voice processing component need not be performed by an ADPCM module but may be a subsidiary part of DSP processing component.            DSP processing component: Digital signal processing.            Micro controller component: Controls all the other components.                        D. The clock 3 provides a single time base for the entire system.        E. Peripherals 7 may include (others may be provided too, as is known to persons skilled in the art):                    A Keypad/touch screen or other input unit            A Microphone            LCD (Liquid Crystal Display)            EPROM (Electrically Programmable Read Only Memory, i.e. a permanent memory for phone book, date, clock . . . )            Sensors.                        F. The Memory 4 (temporarily) stores the data processed by microprocessor 5 or TDMA processor 2.        G. The Power Supply System 6 may be supplied by a (rechargeable) battery 25 and produces power to the whole system.        
There are communication links between the Radio Frontend 1 and the TDMA processor 2, between the TDMA processor 2 and the microprocessor 5, and between the microprocessor 5 and the peripherals 7. Moreover, both the TDMA processor 2 and the microprocessor 5 are connected to memory 4. The clock 3 and the main system supply 6 are connected to all other components.
A. A. Milani, S. Sheikhaei, “Implementation of a Baseband Processor for DECT Cordless Telephone using an ADSP-2186”, SHARC International DSP Conference 2001, pages 268-273 disclose a DECT system with a Power Management block monitoring activity of other blocks for reducing power consumption. To that end, the Power Management block, in the idle mode, turns off other blocks.
WO98/58460 discloses a calibrator for a mobile station of a TDMA wireless communication system such that the calibrator calibrates a low-frequency clock to a high frequency clock locked to the system timing. When the mobile station is in idle mode, the control processor of the mobile station commands the mobile station to enter into sleep mode to minimize power consumption. During sleep mode, only the calibrated low-frequency reference clock remains operating to clock the sleep logic.
At the end of the sleep period, the high-frequency clock and the TDMA timer in the mobile station are powered up. Before the TDMA timer starts up, the station takes a settling period into account to allow system components like oscillators to arrive at a stable oscillating condition. After the settling period, the mobile station automatically starts communicating with the fixed part of the network. Neither a booting action for the mobile station is disclosed nor generating a separate synchronization signal to start communicating with the fixed part of the network.